Add an explanation of the new upstream merging process

The part about conflict resolution requires some expanding. I'm
leaving this to the future for time reasons.

Done as part of AP9-2571

README.md

@@ -119,6 +119,46 @@ directory.
 
 (Original Bromite README follows)
 
+## Merging upstream changes
+
+Periodically the Bromite project will add new patches for
+release. There is a specific process to add these to this repo which
+should be followed to ensure a clean and maintainable Git history.
+
+1. Make sure the upstream remote is established.
+
+If you do not have an upstream remote, add it with:
+
+    git remote add upstream https://github.com/bromite/bromite.git
+
+2. Retrieve the upstream changes
+
+    git fetch upstream
+
+3. Find the latest Bromite release tag
+
+    git describe --tags --abbrev=0 upstream/master
+
+This will give you output like `94.0.4606.94` which we'll use as an example going forward.
+
+4. Create a new branch based on this tag
+
+    git checkout -b upstream-94.0.4606.94 94.0.4606.94
+
+5. Rebase this branch on the current master
+
+    git rebase origin/master
+
+If the rebase complete cleanly, you're done! Push the branch and open
+a PR to master.
+
+If you run into conflicts, there is more work to do. We want to keep
+the upstream commits faithful to their original source. This means
+collecting any conflicts and putting them into a separate commit. The
+procedure is largely the same but start out by adding a commit with
+the conflicts pre-resolved. Then rebase onto `origin/master` and -- if
+the rebase succeeds cleanly -- push the new branch.
+
 # Bromite - Take back your browser
 
 <img src="https://www.bromite.org/release.svg" alt="current Bromite release" title="current Bromite release" /> <img src="https://www.bromite.org/license.svg" alt="GNU GPL v3" title="GNU VPL v3" />